The invention relates to a method and a circuit arrangement for the correction of phase and/or frequency errors in digital multicarrier signals which have been transmitted, in particular, using the OFDM method.
The so-called OFDM (Orthogonal Frequency Division Multiplexing) modulation method is used for transmitting terrestrial digital broadcast radio and television signals, for example in accordance with the DAB or DVB-T Standard. This method makes use of a large number of modulated carriers, which are transmitted using frequency-division multiplexers. This results in various advantages, such as improved bandwidth utilization or a reduction in interference effects from multipath reception. However, one disadvantage of the OFDM method in comparison with single-carrier modulation methods is the necessity to reproduce exactly the frequency and phase of the originally transmitted carrier signals. This causes increased sensitivity to frequency errors and phase noise, and thus places more stringent requirements on the frequency and phase purity of the oscillators used for frequency conversion in the receiver.
The phase and frequency errors can be split into two components. The so-called self-noise component in this case represents the noise component of the respective carrier, which is caused by said carrier itself and is mapped onto itself. The so-called external-noise component is, in contrast, caused by adjacent carrier interference from the so-called leakage effect of the Fast-Fourier Transformation (FFT) used in the receiver for demodulation.
A method by means of which the self-noise component of the error can be estimated and corrected has been disclosed in Robertson P., Kaiser S.: "Analysis of the Effects of Phase-Noise in Orthogonal Frequency Division Multiplex (OFDM) Systems", ICC 1995, pages 1652-1657. In this case, a common rotation phase .psi..sub.e is determined in the receiver, after the FFT, by averaging all the carriers. All the carriers are then shifted back through this error amount, which is also called Common Phase Error (CPE), by multiplication by the factor e.sup.-j.psi.e. This completely, or at least partially, corrects the self-noise component. However, the external-noise component cannot be corrected either by this method or by other known methods, even though this is highly desirable in order to improve the signal-to-noise ratio.